X Corps OpnO No. 1 was dated on the 28th, though not received by Division until the 30th. By that time, Division planning had made so much progress that Embarkation Order 1–50 was issued on the last day of the month, followed on 4 September by the final draft of Division OpnO 2-50. Operations orders of JTF–7 and TF–90 were issued concurrently.
On 30 August, ComNavFE issued his Operation Plan 108–50, assigning to JTF–7, of which X Corps was a part, the mission of seizing by amphibious assault a beachhead at Inchon. X Corps OpnO No. 1 was dated on the 28th, though not received by Division until the 30th.
By that time, Division planning had made so much progress that Embarkation Order 1–50 was issued on the last day of the month, followed on 4 September by the final draft of Division OpnO 2-50.
Operations orders of JTF–7 and TF–90 were issued concurrently.
Essence of order:
Marine aircraft from two escort carriers, naval aircraft from the USS Boxer (CV-21), and British aircraft from a light British carrier would provide as much support aircraft as could be concentrated in and over the landing area, and would be controlled from the amphibious force flagship (AGC) USS Mount McKinley (AGC-7). An arc extending inland thirty miles from the landing site described the task force objective area. [25-26] In order to carry out its various missions, Joint Task Force Seven organized its subordinate parts as follows:
For the naval phases, the command post of Admiral Struble was on the USS Rochester (CA-124); that of Rear Admiral Doyle, second in command, was on the USS Mount McKinley (AGC-7). More than 230 ships were assigned to the operation. Surface vessels of JTF 7 were not to operate within twelve miles of Soviet or Chinese territory nor aircraft within twenty miles of such territory. [25-27]
MacArthur had selected Inch'ŏn as the landing site for one paramount reason: it was the port for the capital city of Sŏul, eighteen miles inland, and was the closest possible landing area to that city and the hub of communications centering there.
Inch'ŏn is situated on the estuary of the Yom-ha River and possesses a protected, ice-free port with a tidal basin. The shore line there is a low-lying, partially submerged coastal plain subject to very high tides. There are no beaches in the landing area-only wide mud flats at low tide and stone walls at high tide. Because of the mud flats, the landing force would have to use the harbor and wharfage facilities in the port area. The main approach by sea is from the south through two channels 50 miles long and only 6 to 10 fathoms deep (36-60 feet). Flying Fish Channel is the channel ordinarily used by large ships. It is narrow and twisting.
The Inch'ŏn harbor divides into an outer and an inner one, the latter separated from the former by a long breakwater and the islands of Wŏlmi-do and Sowŏlmi-do which join by a causeway. The greater part of the inner harbor becomes a mud flat at low tide leaving only a narrow dredged channel of about ~13 feet in depth. The only dock facilities for deep draft vessels were in the tidal basin, which was 1,700 feet long, 750 feet wide, and had an average depth of 40 feet, but at mean low tide held only [???] feet of water. [25-28]
Inch'ŏn promised to be a unique amphibious operation-certainly one very difficult to conduct because of natural conditions. Tides in the restricted waters of the channel and the harbor have a maximum range of more than 31 feet. A few instances of an extreme 33-foot tide have been reported. Some of the World War II landing craft that were to be used in making the landing required 23 feet of tide to clear the mud flats, and the LST's (Landing Ship, Tank) required 29 feet of tide-a favorable condition that prevailed only once a month over a period of three or four days. The narrow, shallow channel necessitated a daylight approach for the larger ships. Accordingly, it was necessary to schedule the main landings for the late afternoon high tide. A night approach, however, by a battalion-sized attack group was to be made for the purpose of seizing Wŏlmi-do during the early morning high tide, a necessary preliminary, the planners thought, to the main landing at Inch'ŏn itself. [25-29]
Low seas at Inch'ŏn are most frequent from May through August, high seas from October through March. Although September is a period of transition, it was considered suitable for landing operations. MacArthur and his planners had selected 15 September for D-day because there would then be a high tide giving maximum water depth over the Inch'ŏn mud flats. Tidal range for 15 September reached 31.2 feet at high and minus .5 feet at low water. Only on this day did the tide reach this extreme range. No other date after this would permit landing until 27 September when a high tide would reach 27 feet. On 11-13 October there would be a tide of 30 feet. Morning high tide on 15 September came at 0659, forty-five minutes after sunrise; evening high tide came at 1919, twenty-seven minutes after sunset. The Navy set 23 feet of tide as the critical point needed for landing craft to clear the mud flat and reach the landing sites. [25-30]
Another consideration was the sea walls that fronted the Inch'ŏn landing sites. Built to turn back unusually high tides, they were 16 feet in height above the mud flats. They presented a scaling problem except at extreme high tide. Since the landing would be made somewhat short of extreme high tide in order to use the last hour or two of daylight, ladders would be needed. Some aluminum scaling ladders were made in Kobe and there were others of wood. Grappling hooks, lines, and cargo nets were readied for use in holding the boats against the sea wall.
The initial objective of the landing force was to gain a beachhead at Inch'ŏn, a city of 250,000 population. The 3d Battalion, 5th Marines, was to land on Wŏlmi-do on the early morning high tide at 0630, 15 September (D-day, L-hour). With Wŏlmi-do in friendly hands, the main landing would be made that afternoon at the next high tide, about 1730 (D-day, H-hour), by the 1st and 5th Marines.
Three landing beaches were selected-Green Beach on Wŏlmi-do for the preliminary early morning battalion landing, and Red Beach in the sea wall dock area of Inch'ŏn and Blue Beach in the mud flat semi-open area at the south edge of the city for the two-regimental-size force that would make the main landing in the evening. Later, 7th Infantry Division troops would land at Inch'ŏn over what was called Yellow Beach.
The 5th Marines, less the 3d Battalion, was to land over Red Beach in the heart of Inch'ŏn, north of the causeway which joined Wŏlmi-do with Inch'ŏn, and drive rapidly inland 1,000 yards to seize Observatory Hill. On the left of the landing area was Cemetery Hill, 130 feet high, on which three dual-purpose guns reportedly were located. On the right, a group of buildings dominated the landing area. The 5th Marines considered Cemetery and Observatory Hills as the important ground to be secured in its zone.
Simultaneously with the 5th Marines' landing, the 1st Marines was to land over Blue Beach at the base of the Inch'ŏn Peninsula just south of the city. This landing area had such extensive mud flats that heavy equipment could not be brought ashore over it. It lay just below the tidal basin of the inner harbor and an adjacent wide expanse of salt evaporators. Its principal advantage derived from the fact that the railroad and main highway to Sŏul from Inch'ŏn lay only a little more than a mile inland from it. A successful landing there could quickly cut these avenues of escape or access at the rear of Inch'ŏn. [25-31]
An early objective of the 1st Marine Division after securing the beachhead was Kimp'o Airfield, sixteen road miles northeast of Inch'ŏn. Then would follow the crossing of the Han River and the drive on Sŏul.
As diversions, the battleship USS Missouri (BB-63) was to shell east coast areas on the opposite side of the Korean peninsula, including the rail center and port of Samch'ŏk, and a small force was to make a feint at Kunsan on the west coast, 100 air miles south of Inch'ŏn.